When an employee leaves your organization, you need to consider what precisely does this employee have access to. This is not an easy task, as in today's world with the proliferation of products, it can be major problem to find out what exactly they do have access to, which can be very tricky as many in business have ‘shared passwords' and it is difficult to know if they know these. A common example here, is the company Facebook account, if in doubt, change it.
For their work accounts, I suggest is that you keep their accounts but promptly change their passwords to a special code that only you know just in case you need to review the information.
Furthermore, you may want to check if something has been left on your computer, we had one client who we discovered had a bit torrent software running. We think that an ex-employee left it on their computer to use as a VPN to download illegal movies.